Many wireless communication systems use integrated circuits to generate and process transmitted and received communication signals. There exists, therefore, a need to convert the electrical signals generated in ICs and on printed circuit substrates to signals appropriate for transmission in air. There is also a parallel need to take signals received by antennas and convert them to signals that may be processed and interpreted by ICs and other circuitry. In the interest of miniaturization and maintaining communication signal integrity, it is desirable to integrate an IC with waveguide, so that waveguide signals may be launched and received directly to and from waveguide. There is a need, therefore, for a practical conversion from a signal travelling in a conductive metal strip or wire directly to a waveguide.
A known conversion is an E-field probe method in which a conductor of a coaxial cable or a coplanar line is positioned on an interior of a waveguide cavity. One end of the waveguide cavity is shorted. Signals in the probe produce an electric field and excite fields in the waveguide that are directly related to the signal. Accordingly, a certain amount of direct coupling can be achieved. Disadvantageously, the E-field probe method of transformation is bandwidth limited and requires complex assembly that is relatively intolerant to manufacturing tolerances due to the importance of the position of the probe in the cavity to achieve maximum coupling.
Another known conversion is disclosed in U.S. Pat. Nos. 2,825,876, 3,969,691, and 4,754,239 and is termed a "ridge transition". The ridge transition comprises a signal line supported by a dielectric substrate and positioned parallel to a ground plane on an opposite side of the dielectric in a microstrip configuration. An end of the microstrip abuts a waveguide cavity and a conducting ridge is positioned at the end of the microstrip and within the waveguide cavity. Although this method produces the desired conversion from microstrip to waveguide, the fabrication, positioning, alignment, and tolerancing of the conducting ridge renders the manufacture and assembly of the part complex and impractical for volume manufacturing.
Another known conversion is disclosed in MTT-S 1998 International Microwave Symposium Digest paper entitled "A Novel Coplanar Transmission Line to Rectangular Waveguide" by Simon, Werthen, and Wolff. The transformer comprises a microstrip line supported by a dielectric substrate. On an opposite side of the substrate, there are two printed conductive patches positioned in a waveguide cavity. The signal travelling in the microstrip induces a current in the patches that is coupled to the other patch. By proper choice of the patch separation constructive interference of the RF signal is achieved in the waveguide. Thereby, launching an electromagnetic wave in the waveguide. Disadvantageously, the structure disclosed has significant insertion loss at higher frequencies and a relatively narrow bandwidth of operation. Although the disclosed design has a simpler structure than the other prior art transformers, it is relatively sensitive to manufacturing tolerances and operating environment. In addition the transition also exhibits higher radiation and thereby reduced isolation and increased loss.
There remains a need, therefore, for a broadband manufacturable microstrip to waveguide transformer for high frequency ICs.